A flyback power supply having as its main purpose the generation of a regulated high voltage should be distinguished from the flyback power supply used in television horizontal deflection circuits. The latter is primarily used to generate in its primary circuit a suitable waveform that will cause a linear sweep of the electron stream across the face of the television's cathode ray tube (kinescope).
Television display tubes use magnetic deflection vice voltage deflection. This eliminates the need for deflection plates inside the kinescope and permits external sweep (deflection) control. The deflection of the electron stream is at right angles to the magnetic field so that the horizontal deflection coil is mounted vertically over the neck of the kinescope forward of the electron gun. A particular waveshape is required to deflect the electron beam linearly, namely, a trapezoidal waveshape with an initial pulse superimposed on its leading edge. This produces a linear current through the horizontal deflection coils, which exhibit a complex impedance to the driving signal. The initial pulse on the leading edge of the trapezoidal waveshape is created by the resonant ringing of the flyback transformer primary, the negative cycle of the sinusoid being cut off by the action of the damper. The energy in the negative cycle is stored in a capacitor and returned to the circuit later in the sweep cycle. Thus, the leading part of the raster scan is controlled by the horizontal output tube and the other part, by the damper tube action. The use of the rapid return sweep, the flyback, to generate the high voltage for the kinescope is a secondary function of the flyback circuit.
In a flyback power supply, the voltage is controlled in many ways. The primary input voltage can be regulated, but this requires a series-pass device which must carry the full charge current and dissipates heat representing a loss of energy efficiency. Shunt regulators across the high voltage output, such as used in color television circuits to keep the kinescope voltage below 25 kilovolts (to prevent x-radiation) also results in the loss of energy efficiency, generating heat and requiring additional expensive devices that can operate at high voltages.
A feedback regulation circuit for controlling the switch in the flyback primary circuit is shown in U.S. Pat. No. 4,016,482--assigned to the same assignee as the present application. The operation of the feedback connection is, however, not explained in detail but is well known in the art.
Another reference, MULTIPLE REGULATOR CONTROLS USING DIVERSION SWITCHES, by H. S. Hoffman, Jr., IBM Technical Disclosure Bulletin, Vol. 21, No. 12, May 1979, pp. 4909-4910, shows two types of regulators for flyback power supplies. There is an overall loop regulator control for regulating one of the output voltages. The loop regulator consists of a pulse width modulator or a frequency controller that is responsive to the difference between the output voltage and a reference voltage. The pulse width or frequency control signal is applied to the switch in the primary circuit so that the first output voltage is maintained at the correct value.
The second output voltage regulator uses a diversion switch for control. In one circuit, the energy is diverted by a conduction-biased transistor to the other voltage output supply and in the other circuit, it is diverted to a short circuit path. By controlling the amount of flyback energy so diverted, the second output voltage is maintained at the desired value.
As will be seen when compared to the invention as described in detail below, the above references do not show, suggest, or teach the invention.